Silane coupling agents have two or more different functional groups on the molecule and generally function as intermediaries that link together organic materials and inorganic materials which do not readily bond with one another. One of the functional groups is a hydrolyzable silyl group which forms a silanol group in the presence of water. This silanol group reacts with a hydroxyl group on the surface of an inorganic material, forming a chemical bond with the inorganic material surface. The other functional group is an organic reactive group, such as a vinyl, epoxy, amino, (meth)acrylic, mercapto or isocyanate group, which forms a chemical bond with an organic material such as any of various synthetic resins. By virtue of these characteristics, silane coupling agents are widely used as, for example, modifiers for organic and inorganic resins, adhesion promoters, and various types of additives.
Among silane coupling agents, those with isocyanate groups have an excellent reactivity with active hydrogen-containing structural groups such as hydroxyl groups, primary and secondary amino groups and carboxyl groups. Hence, such isocyanate group-containing silane coupling agents are used not only in applications as adhesion-improving agents, but also as resin modifiers for introducing hydrolyzable silyl groups onto organic polymers.
A number of methods for efficiently and industrially preparing isocyanate group-containing organosilicon compounds have been described, including typically methods involving the reaction of an amine with phosgene or the thermal degradation of carbamate. Art relating to the former method is disclosed in, for example, JP-B H05-8713, JP-B H05-8714 and JP No. 3806459 (Patent Documents 1 to 3). However, phosgene is difficult to handle on account of its toxicity, and there remain challenges with this approach in terms of productivity, such as the trapping and removal of hydrochloride generated.
Art relating to the latter method is disclosed in, for example, JP-A H10-1486, JP No. 2686420, JP No. 2963309 and JP No. 4778844 (Patent Documents 4 to 7). Because the toxicity is lower and less by-product is generated than in the phosgene method, this is the mainstream approach for small- and medium-scale isocyanate production. However, this method requires the installation of production equipment that efficiently removes alcohol generated at high temperatures and so, from an industrial perspective, there remain challenges with this approach as well.
A common method known for synthesizing silane coupling agents involves a hydrosilylating reaction between hydrosilane and an olefin compound. An example of this method applied to isocyanatosilane synthesis is hydrosilylation using hydrosilane and a polymerizable group-containing isocyanate. However, allyl isocyanate, which is a typical industrially available polymerizable group-containing isocyanate, has a high toxicity. Moreover, being a nitrogen-containing compound, it poisons the platinum complex that serves as the hydrosilylation reaction catalyst. This reaction thus appears to be impractical.
An organosilicon compound in which there remain unreacted isocyanate groups can be obtained even by reacting one mole of a diisocyanate compound that is readily available industrially with one mole of an organosilicon compound having an active hydrogen-containing structural group, such as an aminosilane or a mercaptosilane. However, given the absence of selectivity here in the reactivity of the isocyanate with the active hydrogen-containing structural group, there are also cases in which some of the starting diisocyanate remains. Also, because side reactions arise in which the NH structures in the urea bonds and thiourethane bonds formed by the reaction react with the remaining isocyanate, it is difficult to obtain the target substance alone. Ensuring the stability of the resulting reaction product also remains a challenge.
Silane coupling agents are typically used also as adhesion modifiers for pressure-sensitive adhesives. In pressure-sensitive adhesives used for attaching liquid crystal cells and optical films, for instance, as the liquid-crystal display (LCD) becomes larger in size and wider, a higher adhesion performance is desired. Contrary to initial expectations that LCD sizes of 20 inches and up would be difficult to achieve, display sizes are rapidly increasing. Major manufacturers have hitherto devoted the bulk of their efforts to the production of small panels less than 20 inches in size. Yet, in line with recent trends, they are now aggressively adopting the latest technology and expanding the range of their products to include large panels 20 inches or more in size.
Thus, in various optical films, the trend in the glass used when manufacturing liquid-crystal display panels is toward larger sizes. However, should a defective product arise during initial attachment, making it necessary to remove the optical film from the liquid-crystal cell and to wash and reuse the cell, in cases where a conventional pressure-sensitive adhesive with a high tack strength has been used, not only would removal of the film be difficult on account of the strong adhesive strength, there would also be a strong likelihood of destroying the expensive liquid-crystal cell during such removal. Hence, this greatly increases the production costs.
Therefore, along with the increase in the size of LCD's, efforts continue to be made to develop high functionality pressure-sensitive adhesives endowed with various adhesive performances such as adhesiveness and reworkability. For example, JP No. 3022993 and JP No. 5595034 (Patent Documents 8 and 9) disclose epoxysilane or isocyanatosilane-containing acrylic pressure-sensitive adhesive compositions, the object being to provide polarizers of excellent durability in high-temperature, high-humidity environments.
Also, JP-A 2011-219765, JP No. 4840888 and WO 2010/26995 (Patent Documents 10 to 12) disclose, as pressure-sensitive adhesives endowed with a low initial adhesive strength, excellent reworkability, increased bonding strength under high temperature and high humidity following attachment, and excellent long-term durability, acrylic pressure-sensitive adhesives which include an organosilicon compound having an alkoxysilyl group on a polyether end.
By including such a silane compound, the substrate and the polarizer can maintain a suitable adhesive strength of the degree required in the actual service environment, the adhesive strength does not rise excessively due to heating and the like, and the polarizer can be easily released without damaging the liquid-crystal element.
Also, in what is a recent technical trend, as touch-sensor LCD's become more widespread, designs that place a pressure-sensitive adhesive layer in direct contact with a transparent electrode layer typically made of indium-tin oxide (ITO) have become the norm. A concern in this product design is that the carboxyl group-containing acrylic polymer base commonly used in pressure-sensitive adhesives may corrode the ITO. Therefore, in its place, a technical transition to pressure-sensitive adhesive compositions based on hydroxyl group-bearing acrylic polymers is underway.
However, in pressure-sensitive adhesives having such an acid-free base polymer composition, the effects obtained using the silane coupling agents that have been effective in conventional pressure-sensitive adhesives are limited; silane coupling agents that help to manifest the same or a higher level of performance have not been found.
Given the above, there exists a desire for a method of preparing isocyanate group-containing organosilicon compounds that is both simple and highly versatile. There is also a desire for, in applications as silane coupling agents, the development of an acid-free base polymer-type pressure-sensitive adhesive which strikes a good balance between the initial reworkability and maintaining a high adhesive strength under high temperature and high humidity.